Gently swirling stratospheric currents apparently swept open a minihole in the ozone layer over Western Europe on Tuesday, according to the European Space Agency. The weak winter sun leaking through the hole does not pose a health hazard and scientists believe it will close up as the currents shift.

Ozone, a molecule containing three oxygen atoms, soaks up the sun's burning ultraviolet radiation before it reaches Earth's surface. A complicated series of chemical reactions generally maintains a supply of this high-altitude sunscreen, which is spread around the planet by stratospheric winds. But sometimes the winds twist into vortexes that resemble gigantic hurricanes. These vortexes can block the replenishing flow of ozone: "It is like trying to throw a baseball through a tornado," says Dave Hofmann, an atmospheric scientist with the National Oceanic and Atmospheric Administration in Boulder, Colorado.

Without a fresh supply of ozone, solar radiation triggers reactions inside the vortex that can burn a hole in the ozone layer, as it regularly does over the Antarctic. Alternatively, says Hoffman, the stratospheric currents may simply have brushed ozone away from Europe, in the same way that winter winds clear off some parts of a snow-covered street. "But we don't have enough data to really pin down what is happening," he says.

The last time the European ozone cover weakened as dramatically as it did this week was almost 15 years ago, says Cambridge University atmospheric scientist David Southwood. To try to work out what might be causing this minihole, he and his colleagues are keeping an eye on it with remote-sensing satellites like the European Space Agency's ERS-2. An ongoing study in Sweden is also monitoring the minihole with a specially equipped DC-8 and several balloon-borne experiments.